Alkaline flooding is a water flooding process which relies on generating surfactants in situ to enhance oil recovery. Those surfactants are generated by neutralizing organic acids in crude oil with alkali. Those surfactants accumulate at interfaces within the porous medium. While several mechanisms have been proposed to explain how this works, they all rely on reducing the oil-water interfacial tension (IFT). Targets for alkaline flooding, therefore, include formations with acidic oils (i.e., oils with acid numbers greater than 0.05 mg KOH/g oil). Heavy-oil formations (.degree.API &lt;20) are especially attractive targets.
U.S. Pat. No. 4,493,371, which is incorporated herein by reference for all purposes, shows a process of recovering acidic oil from a formation. The oil is displaced with a mixture of gas and an alkaline solution that contains a cosurfactant. The acidic oil reacts with the alkaline solution to form monovalent cation soaps of low interfacial activity. The preferred alkaline component of that patent is NaOH.
There has been much research on the use of alkaline flooding as a follow-up process to steam flooding. Some features of steam-flooded, heavy-oil formations which make them attractive for alkaline flooding include: (1) the high interfacial activity of heavy oil against alkaline solutions, (2) the large amount of oil left in the formation after steam flooding, (3) the higher mobility of the oil caused by prior heating, (4) the small well spacing (i.e., less alkali residence time), (5) the shallow formation depth (i.e., lower well costs), and (6) the high permeability and low reservoir pressure (i.e., high injectivity). Drawbacks include high alkalinity losses and problems with controlling mobility in a nonisothermal reservoir.
Perhaps the most serious obstacle is high alkalinity losses. These losses may occur by ion exchange, precipitation, and rock dissolution. Of these three mechanisms, rock-dissolution appears to be the most severe. The dissolution reaction appears to follow first-or second-order kinetics with respect to hydroxide concentration. Therefore, raising the pH of the alkaline solution may not improve its transport through the formation.